1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, using an electrophotographic system or an electrostatic recording system, and particularly to a so-called cleaner-less system in which residual toner remaining on an image bearing member is collected by a developing device for re-use.
2. Related Background Art
In recent years, image forming apparatuses, such as copying machines, have been reduced in their sizes, but limitations to the down-sizing are likely to occur if only units for performing electrification, exposure, development, transfer, fixation, and cleaning are down-sized.
Further, in an image forming apparatus of a type in which a toner image formed on a photosensitive member serving as the image bearing member is transferred to a recording material or the like, residual toner remaining on the photosensitive member after the transferring process is collected by a cleaner, and becomes a waste toner. Such disposal is, however, not preferable from the standpoint of an environmental aspect.
Accordingly, there has been proposed a so-called cleaner-less system in which residual toner remaining on a photosensitive member is charged by a charging roller serving as a charging unit for charging the photosensitive member, and is collected by a developing unit for re-use.
In the image forming apparatus using such a cleaner-less system, the residual toner remaining on the photosensitive member after the transfer is simultaneously charged in the same electrical polarity as the photosensitive member when the photosensitive member is electrified by the charging roller. Further, collection of unnecessary toner on the photosensitive member is performed by electrostatically collecting toner attached to a non-image portion in an image zone by the developing unit when an electrostatic image on the photosensitive member is developed. Here, the charging roller is rotated in the same direction as a rotational direction of the photosensitive member.
FIG. 2 schematically illustrates a manner in which the residual toner on a photosensitive member 1 is charged by rotation of a charging roller 20 in the same direction as the rotational direction of the photosensitive member 1. A residual toner 14 remaining on the photosensitive member 1 is brought into contact with the charging unit 20 at an upstream end portion A1 along the rotational direction of the photosensitive member 1 in a contact portion N11 between the charging roller 20 and the photosensitive member 1, and is hence electrified. Since the rotational direction of the charging roller 20 is the same as that of the photosensitive member 1, the toner 14 on the photosensitive member 1 is scraped off by the charging roller 20, and moved to the charging roller 20. The toner 14 moved to the charging roller 20 is again rubbed off by the photosensitive member 1 at a downstream end portion A2 along the rotational direction of the photosensitive member 1 in the contact portion N11, and is moved to the photosensitive member 1 once again.
Further, the toner 14 on the charging roller 20 is electrostatically reciprocated between the charging roller 20 and the photosensitive member 1 in the vicinity of the contact portion N11 by an alternating AC voltage applied to the charging roller 20. Accordingly, the toner 14 on the charging roller 20 can be readily moved to the charging roller 20, or the photosensitive member 1. At this time, the toner 14 is not present in the contact portion N11. Therefore, the charging roller 20 is brought into direct contact with the photosensitive member 1, and hence the photosensitive member 1 is appropriately electrified.
However, in cases where the amount of the residual toner 14 remaining on the image bearing member 1 is large, such as a case where an image ratio is high in the image zone of a previous image, and an image using a plenty of toner is transferred on a recording medium, it is difficult for the above-discussed charging roller 20 to electrify the toner 14 on the photosensitive member 1 to a sufficient degree.
In other words, much residual toner is accumulated near the end portion A1, and a portion of the residual toner 14 is rubbed off from the photosensitive member 1 to the charging roller 20 without being in direct contact with the charging roller 20. On such an occasion, the toner 14 without direct contact with the charging roller 20 is not charged to a sufficient degree. Such non-electrified residual toner 14 is not collected by the developing device. Thus, the problem that the toner remains on the non-image portion in the image zone is liable to occur.